Adequate and reliable braking for vehicles, particularly for large tractor-trailer trucks, is desirable. While drum or disc wheel brakes are capable of absorbing a large amount of energy over a short period of time, the absorbed energy is transformed into heat in the braking mechanism.
Braking systems are known which include exhaust brakes which inhibit the flow of exhaust gases through the exhaust system, and compression release systems wherein the energy required to compress the intake air during the compression stroke of the engine is dissipated by exhausting the compressed air through the exhaust system.
In order to achieve a high engine-braking action, a brake valve in the exhaust line may be closed during braking, and excess pressure is built up in the exhaust line upstream of the brake valve. For turbocharged engines, the built-up exhaust gas flows at high velocity into the turbine of the turbocharger and acts on the turbine rotor, whereupon the driven compressor increases pressure in the air intake duct. The cylinders are subjected to an increased charging pressure. In the exhaust system, an excess pressure develops between the cylinder outlet and the brake valve and counteracts the discharge of the air compressed in the cylinder into the exhaust tract via the exhaust valves. During braking, the piston performs compression work against the high excess pressure in the exhaust tract, with the result that a strong braking action is achieved.
Another engine braking method, as disclosed in U.S. Pat. No. 4,395,884, includes employing a turbocharged engine equipped with a double entry turbine and a compression release engine retarder in combination with a diverter valve. During engine braking, the diverter valve directs the flow of gas through one scroll of the divided volute of the turbine. When engine braking is employed, the turbine speed is increased, and the inlet manifold pressure is also increased, thereby increasing braking horsepower developed by the engine.
Other methods employ a variable geometry turbocharger (VGT). When engine braking is commanded, the variable geometry turbocharger is “clamped down” which means the turbine vanes are closed and used to generate both high exhaust manifold pressure and high turbine speeds and high turbocharger compressor speeds. Increasing the turbocharger compressor speed in turn increases the engine airflow and available engine brake power. The method disclosed in U.S. Pat. No. 6,594,996 includes controlling the geometry of the turbocharger turbine for engine braking as a function of engine speed and pressure (exhaust or intake, preferably exhaust).
In compression-release engine brakes, there is an exhaust valve event for engine braking operation. For example, in the “Jake” brake, such as disclosed in U.S. Pat. Nos. 4,423,712; 4,485,780; 4,706,625 and 4,572,114, during braking, a braking exhaust valve is closed during the compression stroke to accumulate the air mass in engine cylinders and is then opened at a selected valve timing somewhere before the top-dead-center (TDC) to suddenly release the in-cylinder pressure to produce negative shaft power or retarding power.
In “Bleeder” brake systems, during engine braking, a braking exhaust valve is held constantly open during the entire engine cycle to generate a compression-release effect.
According to the “EVBec” engine braking system of Man Nutzfahrzeuge AG, there is an exhaust secondary valve lift event induced by high exhaust manifold pressure pulses during intake stroke or compression stroke. The secondary lift profile is generated in each engine cycle and it can be designed to last long enough to pass TDC and high enough near TDC to generate the compression-release braking effect. Such a system is described for example in U.S. Pat. No. 4,981,119.
The present inventor has recognized the desirability of an alternate design solution that would deliver improved engine braking at a reduced cost.